The present invention relates in general to fabrication methods and resulting structures for semiconductor devices. More specifically, the present invention relates to fabrication methods and resulting structures for semiconductor memory devices having vertical active regions.
As the functionality of integrated chips increases, the need for more memory also increases. Designers have been looking to decrease the size of the memory elements and to stack more memory elements in a unit area to achieve greater capacity and lower costs per bit. With advancements in lithographic techniques, significant shrinkage of memory elements has been achieved. For example, flash memory can achieve high density by using 3D arrays, such as using vertical NAND cell stacking. Due to its high density, flash memory has been widely used as a large-capacity, nonvolatile memory, which can store data when it is powered off. However, it has been found that further miniaturization of the flash memory is limited as production costs have become increasingly high.
Designers are now looking at next generation nonvolatile memories such as MRAMs (Magnetoresistive Random Access Memory), PCRAMs (Phase Change Random Access Memory), CBRAMs (Conductive Bridging Random Access Memory) and RRAMs (Resistive Random Access Memory), to increase writing speed and decrease power consumption. Among the nonvolatile memories, the RRAM has the most potential to replace flash memories due to its simple structure, simple crossbar array and ability for low-temperature fabrication. A unit element of the RRAM is a two-terminal device composed of only an insulator positioned between two metal electrodes. For example, in a RRAM, a memristor element (e.g., HfOx) is sandwiched between two electrodes. Defects (e.g., oxygen vacancies) are intentionally introduced in the memristor film that can be programmed to different logic states such as a low-resistance state (logic “1”) or a high-resistance state (logic “0”) by changing the polarity of electrical field across the memristor element.